GOMOS measures atmospheric constituents by spectral analysis of the spectral bands between 250 nm to 675 nm, 756 nm to 773 nm, and 926 nm to 952 nm. Additionally, two photometers operate in two spectral channels; between 470 nm to 520 nm and 650 nm to 700 nm, respectively.
As shown in the figure below, the instrument line of sight can
be successively oriented towards stars and maintained whilst the
star is setting behind the Earth's atmosphere observed on the
horizon. During the star occultation, the ultraviolet, visible and
near-infrared spectra of the star are continuously recorded.
GOMOS Measurement Principle
The instrument line of sight can be successively oriented towards a preselected star and maintained whilst the star is setting behind the Earth's atmosphere observed on the horizon. During the star occultation, the ultraviolet, visible, and near-infrared spectra of the star are continuously recorded.
As the star sets through the atmosphere, its spectrum becomes more and more attenuated by the absorption of the various gases in the atmosphere, each of which is characterised by a known, well-defined spectral signature. Back on the ground, these attenuated spectra recorded by GOMOS are compared with the unattenuated stellar spectrum measured a few tens of seconds earlier, outside the atmosphere, so allowing the absorption spectra to be derived very accurately. This radiometrically self-calibrating method is protected from sensitivity drifts and is thus capable of fulfilling the challenging requirement of reliably detecting very small trends in ozone (and other gas) profiles.
During day-side observations, the solar radiation scattered by the atmosphere is superimposed to the star signal as the line of sight starts crossing the atmosphere. In order to be able to retrieve the star signal transmitted through the atmosphere without the background component, the (vertically imaging) spectrometers are recording the background spectrum just above and below the star too. These spectra are then used on ground for background removal.
GOMOS uses, as SAGE, the occultation measurement method which, compared to other instruments, offers the advantage of high measurement accuracy and of very good altitude profiling. However, instead of using the sun as an occulting source, GOMOS uses stars to perform the occultation measurements. There are some 100 stars bright enough for GOMOS to observe as they set through the atmosphere. The figure below shows a typical example of the northern hemisphere coverage of the star occultations available over one day. Over one day/one month there are typically 1600/48000 occultations to be chosen from.
Selection criteria like coverage of specific latitudes/longitudes, altitude ranges, etc. can be applied, while still maintaining a good global coverage. Typically GOMOS performs more than 600/18000 profile measurements per day/month.